A recording method has been known in that the luminous power of an object image formed on a surface of an image-pickup element by the image-pickup element using a photoelectric transducer, such as a CCD (charge coupled device) and a CMOS (complementary metal-oxide semiconductor), is converted into an electric output by the photoelectric transducer so as to record it.
Along with the recent technological progress in micro-fabrication technique, a central processing unit (CPU) is speeded up and memory media are highly integrated, so that high-capacity image data, which have been difficult to handle, have got to be processed at high-speed. Also, light-receiving elements are highly integrated and miniaturized, so that the recording at high spatial frequency is enabled due to the high-integration and the entire camera is miniaturized due to the miniaturization.
However, a problem has arisen in that the light-receiving area of the individual photoelectric transducer is reduced due to the high-integration and the miniaturization, so that the noise effect is increased along with the reduction in electric output. For preventing this, there have been approaches to the increase in luminous power arriving on the light-receiving element by increasing an aperture ratio of an optical system and to the arrangement of a micro-lens element (so-called micro-lens array) adjacent to each element. The micro-lens array limits the exit pupil position of the lens system, although it guides the luminous flux passing between elements adjacent to each other to the element. That is, when the exit pupil position of the lens system comes close to the light-receiving element, the angle of the principal beam arriving at the light-receiving element defined by the optical axis is increased so that the angle of the off-axial luminous flux toward the picture periphery to the optical axis is further increased so as not to arrive on the light-receiving element, resulting in luminous power shortage.
Various inventions have been disclosed about zoom lenses suitable for cameras for recording object images via the above-mentioned photoelectric transducer.
For example, a mainstream zoom lens for a video camera includes a so-called PNPP four-lens group composed of a positive lens group, a negative lens group, a positive lens group, and a positive lens group arranged from the object side in that order. In particular, a zoom type has been mainly used in that during variations, the first and third lens groups are fixed in the optical axial direction; the second lens group functions as a variator; and the fourth lens group functions as a compensator.
Along with recent high-integration of the light-receiving elements, the lens system has been miniaturized and advanced in capabilities. For this end, it is especially essential to preferably correct variations due to aberrations accompanied by lens variations in position.
In the PNPP four-zoom lens group mentioned above, since only one lens group having negative refracting power exists, the negative distortion has been difficult to be corrected at the wide-angle end. As the variator is only the second lens group, the refracting power has been difficult to be reduced for obtaining a predetermined variable power ratio, so that the negative distortion needs to be corrected with the other lens groups. As a result, the third lens group has been composed of a positive partial group and a negative partial group so that the negative distortion being liable to be generated at the wide angle end is preferably corrected and simultaneously the third lens group is constructed to have strong positive refracting power for astringing the luminous flux exhaled from the second lens group.
It is generally known that use of an aspheric lens be effective for compatibility between the miniaturization and the offering high-performance. In the second lens group serving as a variator, with high refracting power, the displacement required for a predetermined variable power ratio is reduced, so that the entire lens length can be reduced. The aberrations generated when the refracting power is increased have been corrected by adopting an aspheric surface.
In the PNPP four-zoom lens group mentioned above, it is known that inventions specifically adopting the aspheric lens to the second lens group include Japanese Unexamined Patent Application Publications No. 08-160299, No. 11-52236, and No. 2002-36554.
In the sixth embodiment of Japanese Unexamined Patent Application Publications No. 08-160299, the second lens group is composed of a biconcave lens and a biconvex lens, and the surface adjacent to the image side of the biconcave lens arranged nearest to the object is aspheric. In the fourth and fifth embodiments of Japanese Unexamined Patent Application Publication No. 11-52236, the second lens group is composed of two negative lenses, and the surface adjacent to the image side of the negative meniscus lens arranged nearest to the object is aspheric. In Japanese Unexamined Patent Application Publication No. 2002-36554, the second lens group is composed of a negative meniscus lens and a cemented lens of a biconcave lens and a positive lens, and the surface adjacent to the image side of the biconvex lens is aspheric.
However, in the second lens group serving as a variator, the off-axial luminous flux passes through off the optical axis at the wide angle end while the on-axial luminous flux passes through in a spreading state at the telescopic end, so that the eccentric coma is liable to develop even by the micro-eccentricity produced during manufacturing, resulting in a problem of the optical quality deterioration.
Hence, as in Japanese Unexamined Patent Application Publication No. 08-160299, when the convex lens surface of the lens arranged nearest to the object of the second lens group opposes an aperture diaphragm, the eccentric coma is liable to develop in the picture periphery at the wide angle end by the micro-eccentricity produced during manufacturing, and because of three single lenses, the eccentric coma is liable to develop in the picture center at the telescopic end by the micro-eccentricity produced during manufacturing, resulting in the difficulty in maintaining predetermined optical quality.
In the zoom lens disclosed in Japanese Unexamined Patent Application Publication No. 11-52236, because a diffraction optical element is included, although the number of lenses is small, the diffraction has been changed by the micro-eccentricity produced during manufacturing, so that the correction state of aberrations is varied, resulting in a problem of the difficulty in obtaining a predetermined optical performance. Hence, a barrel structure and an adjustment method in those the eccentricity produced during manufacturing is suppressed as much as possible are required, so that the barrel structure and the adjustment operation have been complicated, resulting in a high increase in cost.
In the zoom lens disclosed in Japanese Unexamined Patent Application Publication No. 2002-36554, because the convex lens surface arranged adjacent to the object of the biconcave lens having an aspheric surface opposes the aperture diaphragm, the coma due to the eccentricity and developed in the picture periphery is largely varied, so that the optical quality has been deteriorated due to the slight assemble error during manufacturing, resulting in a problem of the difficulty in obtaining stable optical quality.
The present invention solves the problems described above and it is an object thereof to provide a zoom lens and an image-pickup apparatus using the zoom lens capable of achieving stable optical quality by inhibiting an effect of assemble errors during manufacturing.